FR2473210A1 - MECHANISM DETECTING THE RELATIVE POSITION OF A VIDEODISK NEEDLE - Google Patents

Abstract

THE PRESENT INVENTION RELATES TO A MECHANISM DETECTING THE POSITION OF A VIDEODISK NEEDLE. ACCORDING TO THE INVENTION, FIRST AND SECOND ELECTRODES 14, 15 ARE ATTACHED TO A NEEDLE CART; A THIRD ELECTRODE 13 IN FIXED RELATIONSHIP WITH RESPECT TO NEEDLE 11 IS ARRANGED BETWEEN ELECTRODES 14, 15 TO FORM RESPECTIVELY FIRST AND SECOND CAPACITIES VARIING ACCORDING TO THE RELATIVE POSITION OF THE NEEDLE WITH RESPECT TO THE CHARIO; A MEANS 7 APPLIES FIRST AND SECOND SIGNALS VARIING WITH TIME TO THE ELECTRODES RESPECTIVELY, THESE SIGNALS INDUCING BY THE COUPLING MEANS AND THE CAPACITIES OF TWO CAPACITORS, A THIRD SIGNAL TO THE THIRD ELECTRODE INDICATING ITS RELATED POSITION AND THUS NEEDLE IN RELATION TO THE TWO OTHER ELECTRODES. THE INVENTION APPLIES IN PARTICULAR TO VIDEODISKS.

Description

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  The present invention relates to a mechanism capturing videodisk signals and in particular to a mechanism for determining the relative position of a signal reading needle with respect to the whole carriage which gives the needle its movement. of

  radial translation through the disc.

  In some types of video disc systems, a disc is used where the information is pre-recorded by means of geometric variations in tracks or grooves near the surface of the disc. The information is reproduced by means of a reading needle which engages the track or furrow and detects representative geometric variations of the pre-recorded signal. In capacitive type systems, the needle-disk interaction operates to form a time-varying capacitance while the geometric variations of a particular track pass beyond the needle due to the rotation of the disk, this ability time varying part of a tuned circuit for amplitude modulating a carrier frequency. The amplitude modulation is then detected and converted into video and audio signals suitable for reproduction on standard receivers. In

  pressure-sensitive systems, geometric variations

  in the groove apply a time-varying force to the needle, which is mechanically applied to a pressure-sensitive transducer for conversion

in electrical signals.

  In videodisc systems of this type, discs having track or groove densities of from 2,362 to 3,937 per centimeter are typically employed. As a result of such high densities, it is difficult to give the needle a radial translation movement reliably through the disk in a normal restitution. Therefore, the needle is mounted in an assembly of a carriage driven by a motor means to give it its radial translation movement through the disk, generally in synchronism with the rotation L4732i O of the disk. Since the tracks tend to be slightly eccentric, the needle is mounted in the carriage for limited radial movement of the needle relative to the carriage. This relative movement mechanically biases the needle mounting arm relative to its normal position and undesirably affects a needle deflection transducer-which allows the movement to be stopped.

  and other special effects. In order to compensate for this

  the relative position of the needle relative to the entire carriage is monitored, and the translation of the carriage is controlled to maintain the needle mounting arm in generally unsolicited condition and

the needle centered on the track.

  Such a system for monitoring the position of the needle is disclosed in US Patent Application No. 055,976 filed July 9, 1979 and entitled "Stylus Position Sensing Apparatus Video Video Dise Player", assigned to the same Applicant as the present invention. . In this particular invention, the position of the needle a) is detected by establishing a capacitance between a first electrode attached to the carriage and a second electrode in fixed relation to the needle, b) in

  measuring the change in capacity caused by changes in

  the relative proximity of the first and second electrodes to each other, c) detecting the relative amplitude of an oscillating signal applied by the first to the second electrode and d) producing

  a control signal proportional to this applied signal.

  The performance of this system may however be affected by changes in the electronic noise parameters present in the needle-arm-disk system as well as variations in the active gain elements of the system. Changes in parasitic parameters tend to affect the amplitude of the oscillating signal applied from the

  first to the second electrode and consequently cause

  error of the control system, especially if the applied signal is measured against a reference

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  fixed. The present invention is directed to a balanced detection system where two signals of two electrodes arranged on each side of a third electrode in fixed relationship with respect to the switched point, apply complementary signals to the third electrode. The two signals applied to the third electrode are proportional to

  the variable capacitance formed between the electrodes and are-

  arranged to add to zero when the third electrode, and therefore the needle, is at the desired position. The translation of the third electrode relative to the desired position creates a change in the summed signals, the amplitude and the phase indicating

  respectively the extent and direction of this movement.

  The invention will be better understood, and other purposes, features, details and advantages thereof

  will appear more clearly during the description

  explanatory text which will follow with reference to the accompanying schematic drawings given solely by way of example illustrating several embodiments of the invention

and in which: -

  FIG. 1 is a partially schematic view

  and partly in the form of a block diagram of a balanced needle position detection system; FIG. 2 is a graph of the amplitude of the signal as a function of the distance of the movement of the needle for various nodes of the circuit of the system of FIG. 1, the amplitude being indicated on the ordinate and the spatial deviation of the 'needle' X 'with respect to

  center being indicated on the abscissa; -

  FIG. 3 is a partially schematic view

  and partially in the form of a block diagram of a synchronous detector driver circuit; FIG. 4 gives a block diagram of an addition and averaging circuit for the signal processing of the detector; - Figure 5 gives a block diagram of a circuit

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  preferred for producing the oscillating signal and detecting the sensor signal;

  - Figures 6 and 8 show partial views-

  schematically and partially in the form of block diagrams of sensor systems of the balanced position of the needle according to the present invention; and FIG. 7 is a graph of amplitude versus time illustrating applied potentials

  to the detection electrodes of FIG.

  In Fig. 1, a signal reading hand 11 engages a disc 10 to reproduce signals that have been pre-recorded in the tracks placed on the surface of the disc. The signals on the disc are applied by the needle and leads 12 and 21 to the read circuit 22 to produce a frequency modulated or FM signal. The FM signal is subsequently processed by a

  circuit 24 to condition it for overdriving

  25. The needle 11 is attached to an arm (not shown) flexibly mounted on a set of a

  carriage to give, with the needle, a movement of trans-

  radially through the disk, that is, in the direction indicated by "xu in the drawing.

  arm on the entire carriage can be made direct-

  or by means of a removable cartridge mounted in the carriage assembly. The elements 8 represent support members attached to the cartridge or to the carriage assembly and which are arranged on each side of the needle / arm assembly. The support members are fixed, in relatively close proximity to the needle, first and second electrodes 14 and 15. A third electrode 13 is attached to the needle / pin set placed between the first and second electrodes 14 and 15. The third electrode 13 is forced to move according to

  minus the movement of the needle in the "x" direction.

  The electrode 13 may be a separate conducting element fixed to the arm of the needle and electrically connected to the flying conductor 12 to electrically connect the needle to the circuit

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  reading or the electrode 13 may be part of the flying conduct itself, which part moves

usually according to the needle.

  The first electrode 14 and the third electrode 13 constitute the armatures of a first variable-air dielectric capacitor and the second electrode 15 and the third electrode 13 constitute the

  armatures of a second dielectric air capacitor.

  variable, the capacitances of the first and second variable capacitors changing in a substantially complementary manner while the third electrode undergoes movement in the "x" direction relative to the elements 8 fixed to the entire carriage. The first (second) capacitor increases in capacitance while the second (first) capacitor decreases in capacitance according to the relation C = m, or is the permitivity of the air, A is the surface of the electrode 13 parallel and adjacent to the electrodes 14 and 15, d is half of the distance between the electrodes 14 and 15 and x is the distance from which the electrode 13 has moved relative to its central position. For the electrode 13 centered between the electrodes 14 and 15, "x" is equal to zero and the first and second

  capacitors have equal capacitance values.

  A signal source 7 produces a time-varying signal of a generally oscillating nature, which signal may have a regular waveform such as a sinusoidal wave or a square wave, for example, or a shape.

  arbitrary wave. For the present description, the source 7

  will be expected to produce a sinusoidally varying signal. The source 7 applies a first signal, V1, to the electrode 15 by the impedance 17. A circuit 20 also operates on the signal at the output of the source 7 to form another signal V2 which is the complement of the signal V1 and the signal V2 is applied to the electrode 14 by the impedance 16. The complement of a signal is defined here as being a signal of instantaneous polarity inverse to the given signal with respect to a given reference. In the

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  case of a regular oscillating signal of constant frequency, its complement is a similar signal but with a phase difference of 1800. The amplitudes of the signal and its complement, for this application, need not be equal. Only in the case where. physical parameters of the balanced system are identical on both sides of the central electrode and if it is desired a zero point of the needle exactly halfway between the two fixed electrodes, the amplitudes of the signal and

  of its complement must be equal.

  The signals V1 and V2 applied to the electrodes 14 and 15 are algebraically added to the electrode 13 by the coupling of the first and second capacitors. Under the condition where the amplitude of the signal V1 is equal to that of the signal V2, the sum V3 of these signals is equal to zero for the electrode 13 arranged equidistant from the electrodes 14 and 15. While the electrode 13 deviates from the central position and approaches the electrode 14 or the electrode 15, the potential V3 decreases in amplitude and takes the phase angle of the signal of the most electrode. close. The amplitude and phase of the potential V3 indicate the extent and direction of movement of the needle 11

  relative to the fixed elements 8 (see Figure 2).

  In FIG. 2, the potentials V1 and V2 are of constant amplitude but in phase opposition and are not affected by the position of the needle. The sum, V3, of this part of the signals V1 and V2 which is applied to the electrode 13 is zero for a zero deviation from the central position, the amplitude increasing for deviations of the needle on each side of the central position. The sum V3 is supposed to be in phase with V1 to the left of the center and in phase with V2 to the right of the center. The sum of V2 + V3 at the central position-is therefore equal to V2 because V3 is zero, decreasing to the left of the center because V2 and V3 are out of phase by 1800 and increasing to the right of the center because V2 and V3 are phase. The curves V3 and V2 + V3

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  are represented as being linear in FIG. 2, a condition obtained when the reading circuit 22 is linear or does not vary with the voltage. On the other hand, if the circuit 22 is of the type having non-linear input characteristics, then the curves V3 and V2 + V3 will consequently have nonlinearities and a

  means may be required to compensate for these nonlinearities.

  [Note that V3 and V2 + V3 are not in fact linear because of the capacitance relation 1 / x, but for small deviations of the central electrode, the capacitance change approaches a functional relationship linear

with the distance x].

  The signal V3 after processing by the read circuit 22 is actually a composite signal comprising the signal FM, representative of the pre-recorded signal on the disk, combined with the sum of the signals V1 and V2. This signal composed at the terminal 23 is applied to the bandpass filter 26. The filter 26 is designed to pass to its output terminal 27, only the components of

  frequency that can be assigned to signals V1 and V2.

  The signal at terminal 27 is applied to a processing circuit 29 where it is detected and damped to modulate the

  potential applied to the drive motor 31 of the carriage.

  Fig. 3 shows a particular circuit 29 'for performing the function of the drive processing circuit 29 of the motor. This circuit consists of a

  synchronous detector and shock absorber / stage of attack 40.

  The damper / driver 40 in response to a DC potential applied to its input terminal 42 by a source 43 produces a nominal signal at its output 30 to drive the motor 31 to drive the trolley

  radially across the disc substantially synchronously

  me with the rotation of the disc. The signal at the output 30 of the device 40 is subjected to an increase or a decrease according to a correction signal applied to a second input terminal 41. The synchronous detector comprises a transistor switch 36 which is open and closed in a manner.

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  phase with one of the signals applied to the electrodes 14 or 15. This signal is applied to the terminal 28 and is conditioned by an amplifier 37 to excite the transistor control electrode for fast transitions between conduction and non-conduction . The added signal V3 is applied to the terminal 27 of where it is selectively

  applied by the transistor switch 36 to the combination

  its capacitor 39-resistance 38. For the closed switch 36, the capacitor 39 is charged in potential according to the potential appearing at the terminal 27. For the open switch 36, the capacitor partially discharges through the resistor 38. The potential resulting in capacitor 39, which is applied to terminal 41, tends towards the average value of the half-wave signal V3. The resulting potential is positive if the signals applied at terminals 27 and 28 are in phase and negative if they are not in phase. Thus, the synchronous detector can produce bidirectional signals to control the stage 40. Conversely, the capacitor 39 and the resistor 38 can be reduced to a prescribed reference potential other than the reference of the ground, in which case the potential at the terminal 41 will vary around the prescribed reference

to control the floor 40.

  FIG. 4 is a diagram of another detection circuit 29. In this circuit, an amplifier 49 adds the signals applied to terminals 27 'and 28 producing an AC output potential at terminal 50 which is expressed by V 50 = V27.R47 / R45 + V28.R47 / R46 where V50, V27 and V28 are respectively the amplitudes of the signals at the terminals 50, 27 and 28 and R45 R46 and R47 are the respective values of the resistors, 46 and 47. If the V3 signal applied to the terminal 27 is zero, which corresponds to a needle at its position

  centered, the potential at the output terminal 50 of the amplifier

  The indicator 49 is proportional to the signal V1 or V2 applied to the terminal 28 and is assumed to have a substantially constant nominal amplitude. This signal is detected by the

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  diode detector comprising a diode 52, a capacitor 53 and a resistor 54 and then it is damped by a

  circuit 51 before being available at the output terminal 30.

  While the needle is displaced from the central position, the amplitude of the AC signal at terminal 50 is modulated by the signal applied to terminal 27. If the signals at terminals 27 and 28 are in phase , the amplitude of the signal at the output terminal 50 increases above the nominal value and decreases when they are out of phase. The DC output potential at terminal 30 increases and decreases with increasing or decreasing amplitude of the AC signal at terminal 50 represented by curve V 2 + V 3 in FIG. 2. A DC potential applied to the direct input terminal of the amplifier-49 by the source 48 allows the adjustment of the nominal potential in

  DC current appearing at the output terminal 30.

  Figure 5 illustrates how the oscillator 7 and the processing circuits can be made using a commercial integrated circuit such as the Motorola MC1357 or RCA Corp. balanced product detector and FM limiter. CA2111A. The numbering in the figure conforms to the connector pin inmos for a standard 14-pin dual-line plastic-in-line assembly. It will be noted that the product detector is used as a synchronous detector with a first input signal V3 derived from the read circuit and a second input signal taken from the limiter amplifier connected to a ceramic filter F to form a

  oscillator useful for driving electrodes 14 and 15.

  FIG. 6 shows a second embodiment of a balanced position sensor where the signals

  oscillation applied to the two fixed electrodes of

  detectors have the same phase but their

  amplitudes are modulated asymmetrically so that

  complementary. The application of signals in phase to the fixed electrodes C47321 C 76 and 78 has the consequence that the components of

  signals applied to the third electrode 77 does not

  not zero for the needle at its zero position.

  To perform a null signal, To obtain a null signal, the applied signals are arranged so that the average,

  over time, the signal added to the third elec-

  trode 77 equals zero. The average time of the sum of the signals is detected in this arrangement

  rather than the absolute value of the sum of the signals. -

  The modulation of the amplitudes of the signals applied to the first and second electrodes is effected at these first and second electrodes by first and second variable impedance means: in voltage respectively connected to the electrodes and a reference potential point. The variable impedance means are arranged in such a way that the impedance change effected by the voltage of the

  first variable impedance is complementary to the change

  impedance by the voltage of the second variable impedance, i.e. the value of the first variable impedance increases concurrently with a decrease in the second variable impedance and the

  value of the first variable impedance decreases

  with an increase in the value of the second variable impedance. The variable impedances can be in the form of capacitances, resistances and others, and they are chosen so that their impedance values do not

  do not load the needle-disk-read circuit system.

  In Fig. 6, an AC signal from the voltage source 61 is applied by a potentiometer 63 and a resistor 65 to the fixed electrode 76 of the capacitor, and is applied by the potentiometer

  63 and a resistor 67 at the fixed electrode 78 of the condensa-

  The potentiometer 63 is used to adjust the signals to the electrodes 76 and 77 to produce a null signal at the center electrode 77 when that electrode 77 is at the desired position. A first variable impedance with the voltage, or VVI 69 has a first end connected

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  by the connection 68 to the fixed electrode 76 of the capacitor and its second end is connected by a potential of

  polarization 70 at a reference potential at terminal 71.

  A second variable impedance with the voltage VVI 73, similar to the impedance 69, has a first end connected by the connection 74 to the electrode 78 of the capacitor

  and a second end connected by the potential-polari-

  72 to the reference potential at terminal 71. VVI 73 is biased oppositely to VVI 69, so a potential change applied by source 61 creates changes

  additional impedance in WVI 73 and VVI 69.

  VVI 69 and resistor 65 form a voltage divider bridge which establishes the AC potential at electrode 76 as a function of the voltage applied at connection 64 or V76 = V64 (Z69 / (Z69 + R65)), where V76 and V64 are respectively the potentials at the electrode 76

  and at connection 64 while Z69 and R65 are respec-

  the impedance and resistance values of VVI 69 and resistor 65. Similarly, the potential V78 to

  the electrode 78 is given by V78 = V66 (Z73 / (Z73 + R67)).

  By way of example, it is considered that the potential V64 is equal to the potential V66 and that the potential signal at the output of the source 61 is a sinusoidal waveform (waveform V62 in FIG. 7). The positive half-waves of the waveform appearing at connection 68 force the impedance of VVI 69 to increase and the negative half-cycles cause the impedance to decrease, so the potential V76 is a higher proportion of V64 for positive half-waves than for negative alternations. Conversely, the V78 potential is a higher proportion of V66 for the negative half-waves than for the positive half-waves. This effect is illustrated by the waveform (b) of Figure 7 where it can be seen that the peak potentials for alternating alternations

  of V76 and V78 are compressed and expanded.

  Considering the central electrode 77 equi-

  distant from the electrodes 76 and 78 and that the potentials V76--

  rf47321 and V78 which appear therein have similar amplitudes but waveforms asymmetrical with respect to their polarity, the average value of the potential applied to the electrode 77 is zero, that is to say that the average of the the sum of the compressed alternations is zero and the average of the sum of the non-expressed halfwaves is zero. If the control electrode is moved from the

  central or zero position, the average value of the signals-

  applied to the electrode 77 increases or decreases. We now consider the case where the electrode 77 approaches the electrode 78. In this case, a higher proportion of the signal V78 and a smaller proportion of the signal V76 are applied to the electrode-77. Since the average value of the signal V78 itself is negative and the sum of the signals V78 and V76 when they are also applied to the electrode 77 is just zero, the condition for the electrode 77 approaching the electrode 78 must produce

  an added signal having a negative average value.

  On the other hand, since the signal V76 itself has a positive average value, the consequence of the displacement of the electrode 77 coming close to the electrode 76 is the production of an added signal having a positive average value. The value of the average signal is thus indicative of the amount of displacement of the electrode 77 and the

  polarity is indicative of the direction of this displacement.

  The polarization potentials 70 and 72 are included where applicable to condition the variable impedance with the particular voltage for operation in

  the desired region of the operating characteristics.

  The detector 87 of FIG. 6 is of the type which

  determines or responds to changes in the average value.

  For example, a low-pass filter will perform the function but its response will be slow. A balanced synchronous detector or product detector as in Figure 5 is more

desirable.

  A second mode usable for the circuit of the

  Figure 6 can be obtained by arranging the VVI devices 73-

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  and 69 and the variable air dielectric capabilities to operate directly with the readout circuit to produce a control or adjustment signal proportional to the position of the needle. In this mode, the read circuit is not responsive to the signal itself from the source 61, i.e. a time varying voltage, or its modulated components applied to the third electrode. The read circuit is of a type producing a signal in response to the instantaneous impedance appearing at its input. This arrangement is protected by the generation of a control signal having an absolute zero rather than a zero in average over time as in the foregoing. FIG. 8 shows a particular embodiment employing this technique, the operation of which is generally the second mode of the position detection circuit. In FIG. 6, the reference 75 designates fixed elements, the reference 80 designates a disc, the reference 82 designates circuits for reading signals, the reference 84 of the audio / video circuits, the 85 mark a television set, the 86 mark a Low-pass filter, the 88 mark an attack stage and the 89 mark an engine

  driving the carriage, the mark 79 indicating the needle.

  The arrangement of FIG. 8 is a particular application of a balanced sensor or detector system employing varactor diodes as variable impedances with voltage in a capacitive sensor type videodisc system. In FIG. 8, the circuit 200 surrounded by the dotted line represents a particular signal sensor circuit that cooperates with the capacitance

  needle-disk 143 to restore the pre-recorded signals

  The capacitor 143 is the effective capacitance formed between the needle 144 and the disc, and varies according to the geometric pattern in the groove of the disc moving beyond the needle. The capacitor 143 is effectively connected in parallel with a capacitor 114 and an inductor 115 to form a resonant circuit in parallel. A coil 116 a47321U driven by an oscillating potential source 117, such as a 915 MHz sine wave, inductively applies a signal to the oscillating circuit at a frequency slightly greater than or slightly less than the nominal resonant frequency of the oscillating circuit. More precisely, the signal of the source 117 intercepts the amplitude-amplitude curve.

  frequency of the oscillating circuit at half of its peak value. Changes in the capacitance value of the capacitor 143 due to the recorded signal, modify the resonant frequency of the oscillating circuit, forcing the oscillating signal applied thereto to be amplitude modulated according to the pre-recorded signal. The amplitude modulated oscillating signal is taken to the oscillating circuit by a coil 118 and is applied to the detector circuit

  composed of a diode 150 and the combination resistance -

  capacitor 152, 151, respectively, which circuit effectively removes the oscillating signal produced by the source 117 and applies a signal representative of the pre-recorded signal in the disc to the connection 119. This signal is processed by the audio and / or video circuits. 120 for transmission to a standard television, via line 141. On the above system, a balanced needle position sensor is obtained using electrodes 105, 106 and 109 to form position-sensitive capabilities 107 and 108. . A first varactor diode 103 connects the capacitor 108 to the reference potential 101 and a second varactor diode 104 connects

  in series the capacity 107 to the reference potential 102.

  The series connection of the capacitor 108 and the effective capacitance of the varactor diode 103, the series connection of the capacitor 107 and the effective capacitance of the capacitor

  diode varactor 104 and the capacity 143 are all effecti-

  in parallel with the capacitor 114 and can operate to change the resonant frequency of the oscillating circuit. An oscillating signal from a source 113, such as a

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  sine wave at 262 kHz, applies a varying potential

  with time by resistances.110 and 11i, respectively

  At the anode of the varactor diode 104 and the cathode of the varactor diode 103, which time-varying signal causes a modulation of the effective capacitance values of the varactors 103 and 104. The capacitance of the varactor diode 103 increases (decreases) while the capacity of the varactor diode 104 decreases (increases). The total capacitance to which the varactor diodes contribute in the coil 115 of the oscillating circuit remains constant in the condition where the capacitances 107 and 108 are equal and

  the varactors are similar and have a capacity-

  linear voltage. As long as the capacity of the elements

  detecting the position remains constant, there is a con-

  zero output of the signal of the oscillator 113 at the output 119 of the detector 150 and therefore a zero-absolute can

to be realized.

  During a translation of the needle, the central electrode 109 will cause an increase (decrease) in

  the capacity 107 and a concomitant decrease (increase)

  It is considered that the capacity 107 is increased by a translation of the needle to the right. The capacitance 107 increases and the total capacity presented by all the capacitors in the oscillating circuit is greater in time synchronism with the negative half-waves of the signal of the source 113. Conversely, for a translation of the needle to the left, the total effective capacity is greater

  in synchronism over time with positive alternations

  These capacitance modulations effect an amplitude modulation of the signal applied to the oscillating circuit by the source 117 in a manner similar to the disk-induced modulation. The amount of capacitance modulation and therefore Amplitude modulation of the carrier indicate the extent of the translation of the needle and the phase of the signal

  Ultimate indicates the direction of translation.

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  The pre-recorded signals and the translation-induced signals form a composite signal available at the connection 119. The translation-induced signal is then extracted from the signal composed by the bandpass filter 130 and is detected by a synchronous detector 140 for produce a command signal or

  DC adjustment at the output terminal 142.

  The system illustrated in FIG. 8 is not limited to the use of varactor diodes as variable voltage impedance elements. The primary necessity imposed by the system is that the total reactance imposed by the position sensing elements in the oscillating circuit of capacitor 114 and coil 115 remains

  constant for a zero position of the electrode 109.

  The signal from the sources 61 and 113 in FIGS. 6 and 8 respectively can be applied by the fixed electrodes of the capacitors by means of the variable elements with the voltage rather than as indicated on the drawing.

  by simply interchanging VVIs and resistors.

  For example, in FIG. 8, the varactor diode 103 may be interchanged with the resistor 110 so that it is connected between the polarization 101 and the electrode 105, and the varactor diode 103 is connected between the source 113 and the 105, the anode of the varactor diode being connected to the source 113. Similarly, the diode

  varactor 104 and resistance 111 can be inter-

  in which case the cathode of the varactor diode 104 is connected to the source 113. Depending on whether the system employed is signal-sensitive (ie, voltage-sensitive) or impedance-sensitive, there are advantages to the process used to link the source to the variable elements with the

  voltage and / or electrodes of the capacitors.

  On the other hand, it will be noted that variable impedances are not limited to variable type with voltage. For example, variable impedances with current such as saturable reactances can be used.

  in a particular implementation.

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  The balanced detector system described may be

  - to capacitive and pressure sensitive sensor systems. Of course, the invention is not limited to the embodiments described and shown which have been given by way of example. In particular, it includes all the means constituting technical equivalents of the means described and their combinations if they are executed according to its spirit and implemented.

  in the context of protection as claimed.

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Claims (9)

R E V E N D I C A T I 0 N S   1.- Mechanism detecting the position of a videodisk needle, characterized by: first and second electrodes (14, 15) firmly attached to a carriage carrying said needle (11); a third electrode (13) in fixed relation with said needle and disposed between said first and second electrodes, said first and third and   said second and third electrodes respectively forming   first and second capacitance varying in value according to the relative position of said needle (11) relative to said carriage, said needle being mounted to allow relative movement between it and said carriage; means (7) for applying first and second time varying signals to said first and second electrodes respectively, said first and second signals inducing, by said coupling means and the capacitances of said first and second capacitors, a third signal to said first and second signals; third electrode, indicating the relative position of said third electrode and thus the needle relative to said first and second electrodes.   2. A mechanism according to claim 1, characterized by a detecting means (29; 29 '; 29 ") responsive to said third signal for producing a substantially DC correction signal proportional to the relative position of the aforementioned needle to effect a change of the speed of the trolley supra.   3. Mechanism according to claim 1, characterized in that the first and second time-varying signals referred to above are sinusoidal waveforms.   of similar frequency but out of phase by 1800.   4.- Mechanism detecting the position of a ú47321 C 19 X   videodisk needle, characterized by: a first electrode (77) in fixed relationship with the needle (79) and forced to move therewith second and third electrodes (76,78) in fixed relation disposed on each side of said first electrode, said first and second electrodes forming a first variable capacitance and said first and third electrodes forming a second variable capacitance; a signal source (61) varying with time; first and second like variable impedance means (69,73); means connecting said first variable impedance means in a first impedance combination in series with said first variable capacitance so that the impedance value of said first variable impedance is synchronously changed by said signal source   varying with time, increasing and decreasing   impedance value with signals at first and second polarities; means connecting said second variable impedance means in a second impedance combination in series with the second variable capacitance so that the impedance value of said second variable impedance is synchronously changed by the signal source   varying with time, decreasing and increasing   impedance with the signals at the first and second polarities; said first and second series impedance combinations being in parallel with respect to said first electrode;   a signal reading circuit (82) responsive to the impedance values; said circuit being connected to said first electrode to produce a setting signal indicating the impedance value of the parallel connection of the first and second series impedance combinations; function of the relative position of said needle   relative to said second and third electrodes.   5. Mechanism according to claim 4, -247321O -   characterized in that said first and second like variable impedances are first and second diodes varactors (103, 104).   6.- Mechanism according to any one of   4 or 5, characterized in that the circuit   signal reading apparatus comprises a resonant circuit (143, 114, 115) in parallel with said first and second series impedance combinations so that said series impedance combinations are varied.   the resonant frequency of the resonant circuit in synchro-   with the signal source varying in time and in a prescribed manner in relation to the relative position of said third electrode.   7. Mechanism according to claim 6, characterized in that the aforesaid reading circuit further comprises: an oscillating voltage source (117) whose signal is connected to said resonant circuit, said signal being modulated in amplitude according to changes in the resonance frequency of said resonant circuit; detector means (150, 151, 152) connected to said resonant circuit for detecting the amplitude modulated signal and producing another signal representing the amplitude modulations;   a synchronous detector for synchronously detecting   said other signal with said signal varying with the weather.   8. A mechanism according to claim 4, characterized in that said needle is attached to an arm mounted loosely on a carriage assembly to provide said needle radial translation through a disk; and in that said second and third electrodes are held in fixed relationship with said carriage assembly proximate said needle and on each side thereof; said first electrode being disposed between said second and third electrodes and having a L473210   fixed relationship with said needle and forming first and second capacitances with said second and third electrodes respectively, the values of said first and second capacitances varying according to the relative position of said needle relative to said carriage assembly; said source producing an AC signal; said first variable impedance comprising a first variable capacitance with the voltage (103) -connected in a first combination in series with said first capacitor (108); said connection means connecting said first variable capacitance with the voltage of the means for applying an alternating signal to increase its capacitance with increasing potentials of an AC nominal donndiddit polarity; said second variable impedance comprising a second variable capacitance with the voltage (104) connected in a second combination in series with said second capacitor (107); said connecting means connecting said second variable capacitance with the voltage of the means for applying an AC signal to decrease its capacitance with increasing potentials of a given polarity of said AC signal; and said reading circuit being responsive to changes the capacitance of said first capacitor series combination with respect to changes in capacitance of said second capacitor series combination to produce the aforementioned adjustment signal indicating the relative value of said first and second capacitors and thus the position of said needle (144) relative to said carriage.   - 9.- Mechanism according to claim 8, characterized in that the aforementioned circuit connected to the first electrode comprises a synchronous detector connected to receive a signal derived from the aforementioned first electrode and further connected to receive a signal of the aforementioned means for applying an AC signal, said L47321 C   synchronous detector producing, at an output terminal, a direct current potential porportionnel to the displacement of the aforementioned third electrode relative to   said first and second electrodes. -